Aminoacyl-tRNA synthetases are key enzymes in the basic translational mechanism and the existence of possible non-classical roles for these enzymes is extremely important for a thorough understanding of the mammalian cell. Chinese hamster ovary cells are especially amenable to these studies because of their proven usefulness as a model mammalian system and the availablity of conditionally-lethal mutants in aminoacyl-tRNA synthetases. The biochemical nature of these lesions is being investigated for the purpose of understanding the nature of the mutation and investigating structure-function relationships in aminoacyl-tRNA particulate complexes. The particulate synthetase complexes are subject to variation as a cell is perturbed, either by mutation or physiologically, with the nature and function of this variation the subject of this investigation.